Process for obtaining metallic tantalum



Patented Apr. 7 1931 omnncn w. BALKE, or HIGHLAND rAnx, rumors, .essrononvrornusrnnnrnon- UCTS COMPANY, INC., OF NORTH Q'HICAGO, ILLINOIS, A

YORK

No Drawing.

This invention relates to the method or process of obtaining Substantially pure metallic tantalum.

The object of this invention is to provide a process by which substantially pure metal-' lic tantalum in a workable form may be obtained by a comparatively few steps which may be readily carried out.

Briefly stated, the first step in the process 10 consists'in electrolyzing a fused salt of tantalum containing a-dissolved oxide of tantalum, using a carbon" (preferably graphite) electrode. It has been found convenient to carry out the electrolyzing process in a furnace consisting of an iron pot which consti tutesthehathod'e, the anode being a relatively large rod of graphite.- 1A rod four or five inches in diameter has been found satisfactory.

In carrying out the present ,process the double salt of tantalum, potassium fluotantalate (K TaF and tantalic oxide (Ta O areemployed. Preliminary to the electrolyzing process it has been found desirable to fuse or melt a portion of the double fluoride. After the double fluoride has become fused, the oxide is added and readily dissolves. The electrolysis is then carried out by passing direct current through the furnace.

It has been found that the double fluoride may be conveniently fused by using an auxiliary graphite electrode and employin alternating current. Thus a portion 0 the double fluoride will first be placed in an iron pot and fused by alternating current and the auxiliary electrode as just stated. As soon as a sufficient quantity of the double fluoride has become fused, a small quantity of the oxide is added and the electrolysis is started by passing direct current through the furnace, the auxiliary electrode being no longer employed.

During the electrolysis the tantalum is produced in the form of crystals which deposit in the pot, building up a sponge-like mass which consists of a mixture of these crystals with tantalum salt. As the mass builds up, the graphite electrode is gradually raised. The electrolysis is continued with the addition, from time totime, of salt PROCESS FOR OBTAINING METALLIC TANTALUH Application filed August 20, 1923. Serial No. 658,289.

and oxide until the pot has become filled, or

oonronA'rIoN or NEW a suificient quantity deposited therein when the mass is permitted to solidify.

This product of electrolysis is a grey or sometimes black mass, consisting largely of the double fluoride of tantailunnsome oxide and the metallic crystals above referred to. The mass also contains more or less graphite which has sloughed off during the electroylsis process; Part of the anode which'has disappeared during the electrolyzing process has united with the oxygen of the oxide and passed off as carbon monoxide or perhaps, thedioxide. If the electrolysis process has been properly carried out, the mass remaining in the'pot at the end of the process will contain in the neighborhood of fifteen per cent (15%) of metallic tantalum. The mass shrinks upon cooling and may be readily removed from the pot.

After themass has cooled, it is ground to a fine powder. This grinding process acts to a great extent to free the metal crystals from the salt with-which the crystals are mechanically combined in the product of the electrolysis. a a

The grinding process may be very conveniently carried out by passing the material through an impact pulverizer, using air separation. By this means the bulk of the salt is removed from the metal crystals. The metal crystals may be further purified by passing the mass over a Wilfley table or by decantation. When run over the Wilfley table the metal crystals which are deposited at one endof the table will be found to be almost ure. of the ilfley table will be found to be largely salt, while the intermediate portion will contain both salt and metal crystals. In certain cases it maybe found desirable to treat the impure metallic powder by passing it over the Wilfley table and also by decantation. It is found that the decantation process is particularly desirable in removing the loose graphite particles from the powder.

The metallic. powder, containing more or less impurities'according to the position on the table where it is delivered, may be treated chemically to assist in purifying it. My

The material at the other end process is .to treat it alternately with a solution of potassium hydroxide and dilute hydrofluoric acid. Sometimes treatment with dilute aquaregia has a beneficial result.

This process partly dissolves the salt and partly loosens it from the metallic particles so that'a few treatments with washing and decantation of the lighter material will leave the metal free from all of the salt and most of the carbon. Incertain instances it may be found preferable to treat the impure metal powder with the hydroxide and acid and employ the VVilfiey table last as a means for further reducing the amount of graphite remaining mixed with the metal powder. In

.other instances it maybe found desirable to employ the Wilfley table both before and after treatin the powder with chemicals.

The chemically treated powder is found to be practically entirely free from salt and to contain less'than one-half of one-percent of carbon. By the use of the Wilfley table the carbon content may be reduced to one-tenth of one-percent, or, in some cases, very much lower. It is to be understood that the order of-the steps, and in fact the entire manner of mechanically separating the metal particles from the rest of the mass may be greatly varied and still et good results. The method above'descri ed has, however, been found to be very satisfactory.

The process thus far described has result ed in producing a metallic powder of tantalum which is substantiall chemically pure and which has mechanica ly mixed with it a very small fraction of carbon. When the process has been properly carried out as above explained, the carbon remaining in the powder is reduced to less than one-tenth of one,-

per cent.

Practicall pure powder may be obtained by double e ectrolysis. In carrying out the second process of the electrolysis a crude tantalum bar may be employed as the anode. It has been found, however, that the anode bar disappears very rapidly during the electrolysis and the amount of metal obtained from the electrolysis does not exceed, and generally is much less than, the amount lost from theanode. Therefore, this process is not desirable and should be used only when it is essential that an absolutely pure tantalum be secured.

In order to obtain a homogeneous bar of tantalum from tantalum powder, the powder is pressed into a bar and is then sintered in a vacuum furnace. In pressing the bar it is desirable to use as high a pressure as possible as the metal grains are relatively coarse, and it has been found that high pressure actually causes the metal grains to flow more or less in the die and a very compactly pressed bar is thus obtained.

After the pressed bar has been formed, it is placed in the furnace and clamped between the upper and lower electrodes of tungsten or molybdenum. One of the electrodes of the furnace must be movable as the bar contracts during the sintering process. The furnace is assembled and exhausted with a suitable air pump and the current turned on and maintained on until the vacuum suddenly drops.

, This first drop in vacuum is probably caused by moisture in the bar which is driven out when the bar is heated. It may be due,

in part at least, to occluded gas. To obtain the most satisfactory result, the current is flashed on in this manner two or three times until the drop in vacuum is not great.

The current is now left on at a relatively low amperage, in the neighborhood of eighthundred amperes for a three-eighths inch square pressed bar, until a vacuum is reached of about one-tenth to two-tenths of a millimeter of mercury. The current used will, of course, vary with the .cross sectional area of the pressed bar. The current is now gradually increased until the bar is heated substantially to its melting point, or probably to,

about 2700-2800. In short, the bar is heated as hot as it safely can be without danger of melting the core of the bar. The

bar is maintained at the high temperature until the vacuum comes back to about onetenth to two-tenths of a millimeter of mercury, when the current is turned off. This process may take one-half to three-quarters of an hour, or it may take much longer. For the three-eighths inch square bar, a current of nearly two-thousand amperes is usually required to bring the bar to the necessary high tem erature. A bar one-half inch by five-eight s inch requires a current of about twenty-eight hundredamperes.

During the sintering process, every increase in temperature causes a further drop in vacuum until, near the end of the sintering process, the vacuum comes back. This is probably due to the bar giving up the gas therein.

The sintered bar thus formed is a homogeneous piece of metal, similar in shape to the pressed bar, but somewhat shorter and is suitable for'rolling or swaging cold. Most of the free carbonremaining mixed with the metal particles forming-the pressed bar passes away during the sintering process,

particularly if a small amount of tantalic----" oxide is added to the metal powder of which the bar is formed.

While in the above description my approved process is described, it is to be understood that this process may be varied slightly in some particulars and still obtain satisfactory results. The invention, therefore, is to be limited merely by the scope of the hereunto appended claims.

Having now described my invention, what I claim as new and desire to secure by Letters Patent of the United States, is:

1. The process .of obtaining tantalum which comprises electrolyzing a. fused bath I containing tantalic oxide and potassium fluotantalate.

2. The process of obtaining tantalum which comprises fusing potassium fluotantalate, adding tantalic oxide to the fused 1o salt, and electrolyzing the fused mass.

3. The process of obtaining tantalum which comprises electrol zing a fused bath of potassiumtantalum uoride and retplacing the tantalum removed from the used bath by adding a tantalum oxide which is soluble in the potassium tantalum fluoride.

4. A process of obtaining tantalum which comprises electrolyzin a fused bath of potassium fiuotantalate an tantalic oxide,

pulverizin the products of the electrolysis, mechanica ly removing the powder from the rest of the mass and alternately treating said poygde'r with potassium hydroxide and an ac1 .5. A process of producing-workable bodies of tantalum'which com rises electrolytically preparing granules o metallic tantalum from a fused bath of potassium tantalum fluoride and a tantalum oxide dissolved therein mechanically and chemically separating said granules from the products of the electrol sis, forming said granules into a body of t e desired size and shape and removing the carbon impurities from said body by sintering.

In witness whereof, I hereunto subscribe my name this 17th da of August, 1923.

. OLAR NCE W. BALKE. 

